#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <stdlib.h>

int main()
{
    int a1, a2, n, d;
    scanf("%d %d %d", &a1, &a2, &n);
    d = a1 + (n - 1) * (a2 - a1);
    printf("%d\n", d);
    return 0;
}